The present invention relates to a twist drill having a tool body formed with spiral grooves or flutes in the outer peripheral surface of the tool body, and in particular, the present invention relates to a twist drill which can smoothly discharge cuttings which are not broken into chips, and which has high rigidity against cutting resistance. The present invention further relates to an improved twist drill having cutting edges which are processed with honing and are processed with a cross-type web thinning on a front flank face of the tool body.
In general, in a twist drill having spiral flutes in which the helix angles are large, the cutting resistance can be decreased since the axial rake angles of cutting edges are large and the cutting performance is increased. However, when the helix angle of the flutes is set too large, the force pushing up chips or cuttings toward the rear end of the flutes is converted to frictional resistance between the chips and the flutes, and the total lengths of the flutes are long, so that the chips tend to cause clogging when stuffed in the flutes. Furthermore, in general, the web taper in which the web thickness is gradually increased toward the rear end of a tool body is adapted to a twist drill. In such a twist drill as well, the cross sections of the flutes are gradually decreased toward the rear end thereof, so that the chips tend to clog when stuffed in the flutes.
Therefore, there is proposed a twist drill which can ensure smooth discharge of chips or cuttings and can control the increase of the cutting resistance, for example, in Japanese Utility Model Application, Laid-Open No. Sho 64-12716. As shown in FIGS. 1 and 2(a) and 2(b) of this Utility Model Application, the twist drill has a cylindrical tool body 1. A pair of spiral grooves or flutes 4 opening to the front flank face 2 of the tool body 1 is formed in the outer peripheral surface 3 of the tool body 1. A cutter bit 7 having cutting edges 6 is soldered at a front wall portion 5 of the flute 4 facing in the rotational direction of the tool body 1. The cutter bit 7 is disposed such that the cutting face 8 is smoothly connected to the wall portion 5 of the flute 4. The front portion of the flute 4, from the front end of the tool body 1 to the cross-section taken along the line 2A--2A, has a constant helix angle to provide a predetermined axial rake angle to the cutting edge 6. The helix angle of the middle portion of the flute 4, in a cross-section taken along the line 2A--2A to the cross-section taken along the line 2B--2B, is gradually decreased from the helix angle at the line 2A--2A to 0.degree.. As shown in FIGS. 2(A) and 2(B), the ratio of the length (L+L) of the flute to the circumferential length of the tool body 1 at the middle portion of the flute 4, in a cross-section taken along the line 2A--2A to the cross-section taken along the line 2B--2B, is larger than the ratio of the length (L+L) of the flute at the front portion of the flute 4 to cross-section taken along line 2A--2A. The rear portion of the flute 4, from the cross-section taken along line 2B--2B to the rear end, is formed in a straight groove having a helix angle of 0.degree., so that the total length of the flute 4 is smaller than in an ordinary twist drill. For this construction, the increase of the cutting resistance can be prevented and the resistance caused due to the discharge of chips can be reduced.
However, in the prior art drill in FIG. 1, the frictional resistance tends to increase since chips rub against the flute and the inner wall of the hole machined by the cutting edge. As a result, the discharge of chips is not smooth and efficient. That is, in the twist drill as above, the width of the flute increases toward the rear end of the flute (in which chips tend to accumulate), and the cross section of the rear end of the flute is large. Therefore, when cuttings are curled at the cutting face adjacent the cutting edge and are broken into chips, the chips can be smoothly discharged through the rear portion of the flute having a large cross section. In contrast, when the cuttings are not broken into chips and are spirally elongated, the cuttings tend to accumulate in the flute. This is due to the fact that the cuttings must flow along a closed space defined by the flute and the inner surface of the machined hole, and the flute has a constant depth through the total length of the flute. This causes the same problem as the prior art drill.
Moreover, in the above twist drill, the rigidity of the tool body is not sufficient since the greater part of the tool body has a wide flute width L and a small cross section, and therefore, the tool body tends to vibrate, and in certain circumstances, the tool body is broken.
As shown in FIGS. 3 and 4, another example of a twist drill is provided. The twist drill has a tool body 11 made of cemented carbide. A pair of spiral flutes 12 are formed in the outer peripheral surface of the tool body 11. A cutting edge 14 is formed at the intersection between the front flank face 13 of the tool body 11 and a wall portion of the flute 12 facing toward the cutting direction of the tool body 11. The front flank face 13 of the tool body 11 is processed with cross-type web thinning, so that two second cutting edges 15 extending from the radially inner end of the cutting edge 14 to the vicinity of the rotational axis of the tool body 11 are formed. Moreover, since the tool body 11 is made of cemented carbide, slight chambers (hereinafter referred as "honing portion") 16 are formed at the cutting edge 14 and the second cutting edge 15 by honing so as to prevent chipping thereof.
In the twist drill constructed as above, there is substantially no chisel edge, which causes an increase of the thrust load and vibration of the tool body when the cutting edge 14 engages a workpiece, and therefore, the stability of the tool body 11 is ensured when the cutting edge 14 is engaging a workpiece and the power consumption for cutting operations can be reduced.
However, in the twist drill, since the second cutting edges 15 formed by the cross-type thinning are formed with the honing portions 16, the honing portions 16 are disposed at a slight distance, and a pin edge portion P having a slight thickness exists at the rotational center of the tool body 11. As a result, the pin edge portion P is easily broken when the twist drill vibrates.